The so-called “Monsanto process” is well known for manufacturing acetic acid by causing methanol and carbon monoxide (CO) to react with each other in the presence of a noble metal catalyst. Originally, this method was developed to utilize a homogeneous catalytic reaction where methanol and carbon monoxide are caused to react with each other in a reaction solution prepared by dissolving a rhodium compound and methyl iodide respectively as metal catalyst and promotor in an acetic acid solvent that also contains water (Japanese Patent Publication No. 47-3334). A modified method was developed to utilize a heterogeneous catalytic reaction with use of a solid catalyst carrying a rhodium compound (Japanese Patent Application Laid-Open No. 63-253047). However, a homogeneous catalytic reaction is not adapted to a high rate of reaction because of the solubility of the catalyst metal is low relative to the solvent so that a large reactor may need to be used as a matter of course. Additionally, water need to be contained in the reaction solution to a certain ratio in order to increase the reaction rate and the selectivity for acetic acid and prevent deposition of the dissolved catalyst and consequently it gives rise to hydrolysis of methyl iodide that is contained as promotor to reduce the yield and corrode the reaction apparatus. For these and other reasons, a method utilizing a heterogeneous catalytic reaction has been developed because it is relatively free from such problems.
Carbonylation of methanol utilizing a heterogeneous catalytic reaction normally involves the use of acetic acid as solvent. More specifically, methanol and carbon monoxide are caused to react with each other under pressure and at high temperature in a reactor in the presence of a solid catalyst carrying a rhodium compound and a promotor of methyl iodide. The liquid reaction product extracted from the reactor is led to a separation system, which typically comprises a distillation means, in order to separate and collect the produced acetic acid, while the residual solution produced as a result of the separation is returned to the reactor. In this stage of operation, a two-phase system or a heterogeneous system exists in the reactor, in which the reaction solution contains acetic acid, methanol and methyl iodide as main components along with particles of the solid catalyst (a three-phase system containing bubbles of CO gas to be more accurate). Note that the reaction solution also contains methyl acetate, dimethyl ether, hydrogen iodide and water, which are reaction byproducts, in addition to the above listed main components. Particles of insoluble resin containing a pyridine ring in the molecular structure and carrying rhodium complex are normally used for the solid catalyst.
A continuous stirring tank reactor (CSTR) adapted to agitate the reaction solution by means of an impeller or a bubble column reactor adapted to agitate the reaction solution by means of bubbles may be used for the carbonylating reaction using a heterogeneous catalyst.
When using a continuous stirring tank reactor, particles of the solid catalyst are agitated and suspended in the reaction solvent and liquid methanol and CO gas are injected from the bottom as reaction raw materials and made to react with each other. Such a continuous stirring tank reactor, or agitation tank type suspension reactor, is accompanied by a problem of an enhanced rate of CO loss because the residence time of CO gas is relatively short in liquid and, once CO exits from liquid to move into the gas phase in the reactor, it can hardly be dissolved into liquid again. It is accompanied additionally by a problem of difficulty of separation of the catalyst and a reduced life span of the latter because it is structurally difficult to take out only the reacting solution from the reactor without allowing the solid catalyst to flow out of the rector and catalyst particles are encouraged to become finer particles by the stirrer.
To the contrary, a bubble column reactor is advantageous because it is free from the above listed problems and, since the reactor is cylindrical, CO gas passing through it can be made to show a long residence time. When such a bubble column reactor is used, the cylindrical reactor is filled with a reaction solvent and a solid catalyst and liquid methanol is supplied from the bottom as reaction raw material, while CO gas is injected upward from the bottom as jet stream. The injected CO gas forms bubbles as it rises in the liquid contained in the cylindrical reactor and particles of the catalyst are also driven to move upward in the cylindrical reactor by the gas lift effect and dispersed into the liquid. As a result, the carbonylating reaction proceeds. Then, the unreacted CO gas and the reaction solution that contains the solid catalyst are separated by a separator arranged at the top of the cylindrical reactor when they got to there. The unreacted CO gas is collected and part of the reaction solution is taken out from the top of the separator as liquid reaction product that does not contain any solid catalyst, while the remaining part of the reaction solution that contains the solid catalyst returns to the bottom of the cylindrical reactor by way of a circulation path by its own weight and is supplied once again to the cylindrical reactor to complete the circulation. With a known method of carbonylating reaction using such a bubble column reactor, CO gas is injected into the liquid contained in a cylindrical reactor as jet stream by way of a nozzle arranged at the bottom of the cylindrical reactor for the purpose of mobilizing particles of the solid catalyst in the reactor (Japanese Patent Application Laid-Open No. 6-340242).
More specifically, in the above reaction step, carbon monoxide is blown into the liquid reaction composition (containing particles of the solid catalyst in the case of a heterogeneous catalytic reaction) in the reactor and the gas phase components including unreacted carbon monoxide are drawn out from the top of the reactor as off gas. The liquid reaction composition that has reacted is separated from the particles of the solid catalyst and drawn out from the reactor so as to be led into a flash column or flash vessel. In the case of a flash column, carbon monoxide and gasified light-fraction components that have been dissolved in the liquid are separated as off gas by means of an operation of flash distillation and the residual liquid composition is divided into a crude acetic acid fraction that is to be refined to produce a final product of acetic acid by way of subsequent steps including a distillation step and a circulating fraction that is to be driven back into the reactor for circulation. In the case of a flash vessel, the liquid reaction composition is divided into a gaseous fraction containing components that correspond to the off gas and the crude acetic acid fraction mentioned above and a remaining liquid fraction, by means of an operation of flash evaporation, of which the gaseous fraction is refined in a subsequent distillation step and the liquid fraction is returned to the reactor. Off gas and a circulating fraction will be produced along with a refined acetic acid fraction, which is a final product, also in the subsequent steps including a distillation step.
As described above, in the process of manufacturing acetic acid, off gas is drawn out in each of the steps of the process including the reaction step and the subsequent steps of separation and refinement. The drawn out off gas contains not only methane and hydrogen that are produced as a result of reaction and unreacted carbon monoxide but also methyl iodide that is a promotor, acetic acid that is used as reaction raw material and reaction solvent and other gasified substances such as methyl acetate. Therefore, conventionally, these useful substances are collected and returned to the reactor before the off gas is burned in an incinerator. A gas absorption operation is generally employed to collect the useful substances from the off gas and the produced acetic acid or the raw material methanol is partly used as absorbent liquid for the gas absorption operation. When the produced acetic acid is partly employed as absorbent liquid, a diffusion step needs to be inevitably provided for the purpose of separating the useful substances absorbed into the acetic acid from the latter after using the latter as absorbent liquid. To the contrary, the use of part of the raw material methanol as absorbent liquid provides an advantage that the methanol that has been used as absorbent liquid can be introduced into the reactor without any treatment. Additionally, while any effort for cooling acetic acid to improve the absorption efficiency is baffled by the relatively high melting point (17° C.) of acetic acid, methanol is advantageous because it is not accompanied by such a problem.